1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium wherein a mixture of a radiation sensitive resin with a partially saponified vinyl chloride-vinyl acetate copolymer containing at least 0.5% by weight of maleic acid, is used as the binder to obtain good dispersibility of the magnetic particles in the magnetic layer, superior surface smoothness of the magnetic layer and superior durability of the medium.
2. Description of the Prior Art
Japanese patent application No. 27529/1980 discloses that when a combination of a radiation sensitive modified resin of e.g. acryl type with a radiation sensitive elastomer is used as a binder for a magnetic recording medium, and the binder is cured and the low molecular weight component therein is polymerized by the application of radiation, the following advantages are obtainable:
(A) The magnetic coating material and the coated magnetic layer have excellent chemical and physical stability, and therefore, they have no particular restriction during the pot life as the coating material or during the surface treatment of the coated magnetic layer until they are subjected to irradiation. Accordingly, by setting up the process so that the irradiation step comes after the necessary processing steps, it is possible to rationalize and automate of the production line and to ensure the quality control. (B) The cross-linking and the polymerization of the radiation sensitive binder take place in such a manner that when the binder is irradiated, radicals will be formed in the binder and they will be instantaneously cross-linked and polymerized, whereby the magnetic layer will be cured and dried. Thus, in order to generate the radicals, it is only required to instantaneously apply a radiation at a level of 20 Mrad at the maximum, whereby the substrate which is commonly made of a polyester film, will not undergo any heat deformation. Further, since the radiation is applied to a sheet material on line, it is possible to avoid a reduction of the yield due to the interlayer transfer of a non-uniform magnetic layer or tightening of the winding which are likely to be caused when the sheet material is subjected to heat curing after being wound-up in a form of a roll, or to prevent a decrease of the S/N ratio in the short wave region which is likely to be caused by a transfer of the surface roughness from the rear surface of the substrate. (C) As mentioned in (B), the reaction is a radical reaction, which does not require heating for a long period of time which used to be required to facilitate the reaction in the case of the conventional heat curing chemical reaction. Further, the cross-linking degree and the polymerization degree can thereby readily be controlled simply by adjusting the dose of the radiation, whereby it is possible to prevent such a trouble as adhesion due to exudation of the low molecular weight component from the magnetic layer. Thus, this process is advantageous also from the aspect of energy conservation in that the heat energy which used to be required for the heat curing step, can be saved. Whereas, in magnetic recording tapes, particularly in magnetic recording tapes for video, uniform dispersibility of the magnetic particles in the magnetic layer and a high degree of the surface smoothness of the magnetic layer are required to obtain a high reproduction output in a short wave region and to improve the S/N ratio. Further, with respect to the reliability and durability, the physical properties which are durable for a long period of time in repeated use, are required. In order to satisfy the above requirements, a number of combinations of various resins have been proposed for the binder for the magnetic layer. By these proposals, certain improvements have been made with respect to the physical properties. However, no adequate improvement has been achieved with respect to the dispersibility and the surface smoothness. For the improvement of the dispersibility, it has been proposed, for instance, to use a surface active agent as the dispersant, or to increase the content of hydrophilic groups (such as a OH group or a COOH group) in the molecule of the binder resin. The former proposal has difficulties such that by the presence of a low molecular weight surface active agent in the magnetic layer, the physical properties of the magnetic layer will be deteriorated or will undergo a change as time passes. In the latter proposal, a certain improvement has been observed, but no adequate dispersibility has been obtained particularly when a substance having poor dispersibility, such as carbon black, is incorporated or when a resin having poor dispersibility is used in combination. In general, if the dispersibility is improved, the surface smoothness will be improved likewise. Even when a resin having poor dispersibility is used, it is possible to reduce the modulation noise attributable to the surface smoothness, by using a substantial amount of a soft resin and conducting surface treatment such as calendering, but there still remains a noise due to the inadequate dispersion. Besides, when a soft resin is used in a substantial amount, the friction of the magnetic layer tends to increase, and the durability will be deteriorated.